Semiconductor, semiconductor device, and method for fabricating the same

ABSTRACT

Method or fabricating semiconductor devices such as thin-film transistors by annealing a substantially amorphous silicon film at a temperature either lower than normal crystallization temperature of amorphous silicon or lower than the glass transition point of the substrate so as to crystallize the silicon film. Islands, stripes, lines, or dots of nickel, iron, cobalt, or platinum, silicide, acetate, or nitrate of nickel, iron, cobalt, or platinum, film containing various salts, particles, or clusters containing at least one of nickel, iron, cobalt, and platinum are used as starting materials for crystallization. These materials are formed on or under the amorphous silicon film.

This is a Divisional application of Ser. No. 08/196,856, filed Feb. 15,1994 pending.

BACKGROUND OF THE INVENTION

This invention relates to a method for obtaining crystallinesemiconductors for use in thin film devices such as thin film insulatorgate type field effect transistors (Thin Film Transistors, or TFTs).

Conventionally, crystalline silicon semiconductor thin films used inthin film devices such as thin film insulator gate type field effecttransistors (TFTs) have been manufactured by forming an amorphoussilicon film on an insulating surface such as an insulator substrate byplasma CVD or thermal CVD and then crystallizing this film in anelectric furnace or the like at a temperature of above 600° C. over along period of twelve hours or more. In order to obtain particularlygood performance (high field effect mobility and high reliability), heattreatment for even longer periods has been required.

However, there have been numerous problems associated with this kind ofconventional method. One problem has been that throughput is low andtherefore costs are high. For example, if 24 hours are required for thiscrystallization process, and if the treatment time for each substrate is2 minutes, it has been necessary to treat 720 substrates at the sametime. However, for example, in a commonly used tube furnace the numberof substrates that can be treated at one time is 50 at the most, andwhen one only apparatus (reaction tube) is used the time taken persubstrate has been as long as 30 minutes. In other words, in order tomake the treatment time per substrate 2 minutes, it has been necessaryto use as many as 15 reaction tubes. This has meant that the scale ofthe required capital investment has been great and that the depreciationon that investment has been large and has kept the cost of the producthigh.

Another problem has been the temperature of the heat treatment.Substrates commonly used in the manufacture of TFTs can be generallydivided into those which consist of pure silicon oxide, like quartzglass, and no-alkali borosilicate glass types, like Corning Co.'s No.7059 (hereinafter referred to as Corning 7059). Of these two classes, inthe case of the former, because they have excellent resistance to heatand can be handled in the same way that substrates are handled inordinary semiconductor integrated circuit wafer processes, there are noproblems relating to heat. However, they are expensive, and their costrapidly increases exponentially along with increases in surface area.Therefore, at present, they are only being used in TFT integratedcircuits of relatively small surface area.

No-alkali glass, on the other hand, is of satisfactorily low costcompared to quartz; however, its resistance to heat is a problem, andbecause its distortion point is generally about 550° to 650° C., and inthe case of particularly easily acquired materials is below 600° C.,with heat treatment at 600° C. problems of irreversible shrinkage andwarping have arisen. These problems have been especially conspicuouswith large substrates of over 10 inches in diagonal. For reasons likethese it has been considered in connection with the crystallization ofsilicon semiconductor films that heat treatment conditions of below 550°C. and less than 4 hours are indispensable to reductions in cost. Anobject of this invention is to provide a method for manufacturing asemiconductor which clears these conditions and a method formanufacturing a semiconductor device in which such a semiconductor isused.

SUMMARY OF THE INVENTION

This invention is characterized in that a crystalline silicon film isobtained by forming a film, islandish film, dot, line, particles orclusters or the like containing nickel, iron, cobalt, platinum orpalladium on or underneath a silicon film in a disordered state of akind which can be described as an amorphous state or a substantiallyamorphous state (for example a state in which portions having goodcrystallinity and amorphous portions exist together) and annealing thisat a temperature which is lower, and preferably 20° to 150° C. lower,than the normal crystallization temperature of amorphous silicon, or ata temperature which is lower than the glass transition point of thesubstrate, for example at a temperature below 580° C.

With regard to conventional silicon film crystallization, methodswherein an island-shaped crystalline film is made to serve as a nucleusand solid phase epitaxial growth is brought about with this as a seedcrystal have been proposed (for example Japanese Laid-Open PatentPublication H1-214110). However, with this kind of method, attemperatures below 600° C. almost no crystal growth progress hasoccurred. Moving a silicon substance from an amorphous state into acrystalline state generally involves a process wherein, with the stateof the substance having been made such that the molecule chains in theamorphous state are cut and these cut molecules do not combine againwith other molecules, these molecules are introduced to molecules havingsome crystalline character and rebuilt into constituent parts ofcrystals. However, in this process a large amount of energy is requiredfor cutting the first molecule chains and maintaining the state whereinthese cut molecules do not combine with other molecules, and this hasbeen a barrier in the crystallization reaction. To provide this energy,several minutes at a temperature of about 1000° C. or several tens ofhours at a temperature of about 600° C. are necessary, and because thetime required varies exponentially in relation to the temperature(=energy), at temperatures below 600° C., for example 550° C., it hasbeen almost impossible to observe any crystallization reaction progressat all. The idea of conventional solid phase epitaxial crystallizationdid not provide an answer to this problem.

The present inventors thought of reducing the barrier energy of theabove process by means of the action of some kind of catalyst,completely separately from the conventional solid phase crystallizationidea. The inventors noted that nickel, iron, cobalt, platinum andpalladium readily combine with silicon and for example in the case ofnickel silicides (chemical formula NiSi_(x), 0.4≦×≦2.5) are formed andthat the lattice constants of nickel silicides are close to those ofsilicon crystals. Accordingly, by simulating the energies, etc, of theternary system crystalline silicon--nickel silicide--amorphous silicon,it was found that amorphous silicon readily reacts at an interface withnickel silicide and that the following reaction (1) occurs:

Amorphous Silicon (silicon A)+Nickel Silicide (silicon B)→NickelSilicide (silicon A)+Crystalline Silicon (silicon B)

(A and B indicate the locations of the silicon)

The potential barrier of this reaction is satisfactorily low, and thetemperature of the reaction is also low.

This reaction formula shows that nickel reconstructs amorphous siliconinto crystalline silicon as it advances. In practice it was found thatthe reaction was started at under 580° C. and observed even at 450° C.Typically, it was shown that crystallization is possible at temperatures20° to 150° C. lower than the normal crystallization temperature ofamorphous silicon. Naturally, the higher the temperature the morequickly the reaction proceeds.

In the present invention, islands, stripes, lines, or dots of nickel,iron, cobalt, platinum or palladium, silicide, acetate, or nitrate ofnickel, iron, cobalt, platinum or palladium, film, particles, orclusters containing at least one of nickel, iron, cobalt, platinum, andpalladium can be used as starting materials. As the above-describedreaction progresses, nickel, iron, cobalt, platinum or palladium expandsaround the starting material, thus enlarging the region of crystallinesilicon. Oxides are not desired materials containing nickel, iron,cobalt, platinum or palladium because oxides are stable compounds andbecause they cannot initiate the reaction described above.

In this way, the crystalline silicon expanding from a certain locationis different from the conventional solid phase epitaxy but hascrystallinity of high continuity. The structure approximates a singlecrystal structure. This is advantageous to utilize semiconductor devicessuch as TFTs. Where a material containing nickel, iron, cobalt, platinumor palladium is dispersed uniformly over a substrate, innumerablestarting points of crystallization exist. Therefore, it has beendifficult to derive a good film of high crystallinity.

The lower the concentration of hydrogen in the amorphous silicon filmwhich serves as the departure material in this crystallization, thebetter were the results (the crystallization speeds) that could beobtained. However, because hydrogen is expelled as crystallizationprogresses, there was not such a clear correlation between the hydrogenconcentration in the silicon film obtained and the hydrogenconcentration of the amorphous silicon film that was the departurematerial. The hydrogen concentration in crystalline silicon obtainedaccording to this invention was typically from 1×10¹⁵ atoms.cm⁻³ to 5atomic %. Furthermore, in order to obtain good crystallinity theconcentrations of carbon, nitrogen and oxygen in the amorphous siliconfilm should be as low as possible, and preferably should be below 1×10¹⁹cm⁻³. Accordingly, this point should be taken into consideration inselecting the material containing nickel, iron, cobalt, platinum orpalladium to be used in practicing this invention.

A feature of this invention is that crystal growth progressescircularly. This is because the nickel of the reaction described aboveadvances isotropically, and this is different than conventionalcrystallization wherein growth occurs linearly along the crystal latticesurfaces.

In particular, by setting the material containing nickel, iron, cobalt,platinum or palladium selectively, it is possible to control thedirection of the crystal growth. Because, unlike crystalline siliconproduced by conventional solid phase epitaxial growth, crystallinesilicon obtained using this kind of technique has a structure in whichthe continuity of the crystallinity over long distances is good andwhich is close to being monocrystalline, it is well suited to use insemiconductor devices such as TFTs.

In the present invention, nickel, iron, cobalt, platinum or palladium isused. These materials are not desirable for silicon which is used as asemiconductor material. If such a material is contained excessively inthe silicon film, it is necessary to remove the material. With respectto nickel, when a growing crystal of nickel silicide arrives its finalpoints, i.e., the crystallization has been completed, as a result of theabove-described reaction, the nickel silicide is easily dissolved inhydrofluoric acid or hydrochloric acid. The nickel contained in thesubstrate can be reduced by treating the nickel with these acids.

In the case where a catalytic element such as nickel, iron, cobalt,platinum and palladium is diffused almost uniformly throughout thesilicon film by the annealing for crystallization, a process wherein thenickel is removed is necessary. To perform this nickel removal, it hasbeen found that annealing at 400° to 650° C. in an atmosphere containingchlorine atoms in the form of chlorine or a chloride is effective. Anannealing time of 0.1 to 6 hours was suitable. The longer the annealingtime was the lower the concentration of nickel in the silicon filmbecame, but the annealing time may be decided according to the balancebetween the manufacturing cost and the characteristics required of theproduct. Examples of the chloride include hydrogen chloride, variouskinds of methane chloride (CH₃ Cl, CH₂ Cl₂, CHCl₃), various kinds ofethane chloride (C₂ H₅ Cl, C₂ H₄ Cl₂, C₂ H₃ Cl₃, C₂ H₂ Cl₄, C₂ HCl₅),and various kinds of ethylene chloride (C₂ H₃ Cl, C₂ H₂ Cl₂, C₂ HCl₃).Especially, the material which can be used most easily istrichloroethylene (C₂ HCl₃). We have discovered by SIMS that preferredconcentration of nickel, iron, cobalt, platinum or palladium in thesilicon film (e.g. a silicon film used for a semiconductor device suchas a TFT) according to the present invention is 1×10¹⁵ cm⁻³ to 1 atomic%, more preferably 1×10¹⁵ to 1×10¹⁹ cm⁻³. At less concentrations, thecrystallization does not progress sufficiently. At higherconcentrations, the characteristics and the reliability deteriorate.

Filmlike bodies containing nickel, iron, cobalt, platinum or palladiumcan be formed using various physical and chemical methods. For examplemethods requiring vacuum apparatus, such as vacuum vapor deposition,sputtering and CVD, and atmospheric pressure methods, such as spincoating and dipping, etc, (coating methods), doctor blade methods,screen printing and spray thermal decomposition.

Spin coating and dipping in particular, while not necessitating grandequipment, are techniques which offer excellent film thicknessuniformity and with which fine concentration adjustment is possible. Assolutions for use in these techniques, acetates and nitrates of nickel,iron, cobalt platinum and palladium, or various types of carboxylic acidchloride or other organic acid chlorides dissolved or dispersed in wateror some type of alcohol (low level or high level), or petroleum(saturated hydrocarbon or unsaturated hydrocarbon), etc, can be used.

However, there was concern that in this case oxygen and carbon containedin those salts might diffuse into the silicon film and cause itssemiconductor characteristics to deteriorate. But, through researchpursued using thermal balancing and differential thermal analysis it hasbeen confirmed that suitable materials break down at temperatures below450° C. to oxides or simple substances and thereafter there is almost nodiffusion into the silicon film. In particular, when substances whichare of lower order like acetates and nitrates were heated in a reducingatmosphere such as a nitrogen atmosphere they broke down at below 400°C. and became the simple metal. Similarly, it was observed that whenthey were heated in an oxygen atmosphere, first oxides were formed andthen at higher temperatures oxygen broke away and left behind the simplemetal.

A crystalline silicon film is fabricated according to the invention, andthis film is used in a semiconductor device such as a TFT. As can beseen from the description made above, a large grain boundary exists atthe ends of a growing crystal where the front ends of the growingmaterial starting from plural points meet. Also, the concentration ofnickel, iron, cobalt, platinum, or palladium is high. For these reasons,it is not desired to form a semiconductor device. Particularly, achannel of a TFT should not be provided in a region having such a largegrain boundary.

The region from which the crystallization starts, that is, the region inwhich the substance containing nickel, iron, cobalt, platinum orpalladium is provided has a large concentration of nickel, iron, cobalt,platinum or palladium. For this reason, attention should be paid to theformation of the semiconductor device. Further, such a region is readilyetched by a solution containing a hydrofluoric acid group as comparedwith a silicon film which does not contain nickel, iron, cobalt,platinum or palladium. For this reason, such a region becomes a cause offormation of a defective contact. Accordingly, where a semiconductordevice is fabricated by making use of the present invention, the patternof a coating containing nickel, iron, cobalt, platinum, or palladiumforming a starting point for crystallization and a pattern of thesemiconductor device must be optimized.

In addition, the present invention provides a process which ischaracterized in that it comprises: forming, on an amorphous siliconfilm or on a film which has such a disordered crystalline state that itcan be regarded as being amorphous (for example, a state which comprisescrystalline portions and amorphous portions in a mixed state), a film,particles or clusters containing at least one of nickel, iron, cobalt,platinum or palladium (which are referred to hereinafter as catalyticmaterials); allowing the catalytic material to react with amorphoussilicon at first, and removing the catalytic material which remainedun-reacted; and annealing the resulting structure at a temperature lowerthan the normal crystallization temperature of a amorphous silicon by,preferably, 20° to 150° C., or at a temperature not higher than theglass transition temperature of the glass material conventionally usedas a substrate, e.g., at 580° C. or lower.

Even after the nickel, iron, cobalt, platinum or palladium atoms areremoved from crystalline silicon, crystallization can be initiated byusing as nuclei the remaining crystalline silicon which was formed bythe reaction (1). As mentioned in the foregoing, the silicon crystalsthus formed by the reaction has excellent crystallinity. Thus, it hasbeen found that the crystallization of amorphous silicon can beaccelerated by using these crystals as the nuclei. It has been shownthat, typically, the crystallization can be effected at a temperaturelower than the normal crystallization of amorphous silicon by 20° to150° C. Furthermore, the time necessary for the crystal growth is foundto be shortened. As a matter of course, the crystallization proceedsmore rapidly with increasing the temperature. A similar reaction, thoughless actively to the case using nickel, is found to occur in the caseusing iron, cobalt, platinum or palladium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(D) show schematically drawn step sequential crosssection structures obtained in a process according to an embodiment ofthe present invention (Example 1);

FIGS. 2(A) to 2(E) show schematically drawn step sequential crosssection structures obtained in another process according to anotherembodiment of the present invention (Example 2);

FIG. 3 shows the result of Raman scattering spectroscopy of acrystalline silicon film obtained in Example 1;

FIG. 4 shows the X-ray diffraction pattern of a crystalline silicon filmobtained in an Example;

FIG. 5 shows the crystallization rate of silicon (Example 2);

FIGS. 8(A) to 8(E) show schematically drawn step sequential crosssection structures obtained in a process for fabricating a semiconductoraccording to a yet another embodiment of the present invention (Example3);

FIGS. 7(A) to 7(C) show a step of introducing a catalyst element using asolution (Example 4);

FIGS. 8(A) to 8(C) are top views of TFTs illustrating crystallizationsteps for manufacturing the TFTs according to the invention and theirarrangement;

FIGS. 9(A-1), 9(A-2), 9(B), 9(C), and 9(D) are cross-sectional views ofTFTs illustrating steps for selectively crystallizing a film accordingto the invention;

FIGS. 10(A) to 10(C) are cross-sectional views of TFTs illustratingsteps of Example 5 of the invention;

FIGS. 11(A) to 11(C) are cross-sectional views of other TFTsillustrating steps of Example 5 of the invention;

FIGS. 12(A) to 12(C) are cross-sectional views of TFTs illustratingsteps of Example 6 of the invention;

FIGS. 13(A) to 13(C) are cross-sectional views of TFTs illustratingsteps of Example 7 of the invention;

FIGS. 14(A) to 14(D) are cross-sectional views of TFTs illustratingsteps of Example 8 of the invention;

FIGS. 15(A) to 15(D) are cross-sectional views of TFTs illustratingsteps of Example 9 of the invention;

FIG. 16 is a graph showing the concentration of nickel in a crystallinesilicon film in Example 9 of the invention;

FIGS. 17(A) to 17(C) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 10 of the invention;

FIGS. 18(A) and 18(B) are cross-sectional views of a substrateundergoing a manufacturing process according to Example 11 of theinvention;

FIGS. 19(A) to 19(E) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 12 of the invention;

FIGS. 20(A) to 20(E) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 13 of the invention;

FIGS. 21(A) to 21(D) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 14 of the invention;

FIGS. 22(A) to 22(D) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 15 of the invention;

FIGS. 23(A) to 23(C) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 16 of the invention;

FIGS. 24(A) to 24(C) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 17 of the invention;

FIG. 25 is a graph illustrating the nickel concentration in acrystalline silicon film; and

FIGS. 26(A) to 26(F) are cross-sectional views of a substrate undergoinga manufacturing process according to Example 18 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in greater detail referring tonon-limiting examples below. It should be understood, however, that thepresent invention is not to be construed as being limited thereto.

EXAMPLE 1

Referring to FIG. 1, a process for fabricating a crystalline siliconfilm by forming a nickel film on a Corning #7059 substrate, andcrystallizing an amorphous silicon film using this nickel film isdescribed below. By using plasma CVD, a 2,000 Å thick silicon oxide film12 as a base film was deposited on the substrate 11, and further thereonan amorphous silicon film 13 at a thickness of from 500 to 3,000 Å, forexample, at a thickness of 1,500 Å. After removing hydrogen from thefilm by keeping the film at a temperature of 430° C. for a duration offrom 0.1 to 2 hours, for example, 0.5 hour, a nickel film 14 wasdeposited thereon by sputtering at a thickness of from 100 to 1,000 Å,for example, 500 Å. A favorable nickel film can be obtained by heatingthe substrate in the temperature range of from 100° to 500° C.,preferably in the range of from 180° to 250° C., because a nickel filmhaving an improved adhesion strength with respect to the silicon filmformed as the base is obtained. A nickel silicide film can be used inthe place of the nickel film.

The resulting structure was then heated in the temperature range of from450° to 580° C. for a duration of from 1 to 10 minutes to allow thenickel film 14 to react with the amorphous silicon film 13, therebyobtaining a thin crystalline silicon film 15. The thickness of thecrystalline silicon film depends on the temperature and duration of thereaction, and a film about 300 Å in thickness can be obtained by areaction performed at 550° C. for a duration of 10 minutes. Theresulting structure is shown in FIG. 1(B).

The nickel film and the nickel silicide film thus obtained from thenickel film through the reaction were subjected to etching usinghydrochloric acid in a concentration of from 5 to 30%. No influence wasfound on crystalline silicon which has been formed by the reactionbetween amorphous silicon and nickel (silicide) by this treatment. Thuswas obtained a structure shown in FIG. 1(C).

The resulting structure was annealed under a nitrogen atmosphere in anannealing furnace whose temperature was kept in a range of from 450° to580° C., for example, 550° C., for a duration of 8 hours. A crystallinesilicon film 16 was thus obtained in this step by crystallizing theamorphous silicon film. The Raman scattering spectrogram and the X-raydiffractogram of the resulting crystalline silicon film are each shownin FIG. 3 and FIG. 4. In FIG. 3, the curve indicated by C--Sicorresponds to the Raman spectrum of a standard sample, i.e., singlecrystal silicon. The curves indicated by (a) and (b) each represent theRaman spectra for a silicon film obtained by the process according tothe present invention, and a film obtained by annealing a conventionalamorphous silicon by the conditions described above. It can be seenclearly from the results that the process according to the presentinvention provides a favorable silicon crystal.

EXAMPLE 2

Referring to FIG. 2, a process for fabricating a crystalline siliconfilm is described below. A 2,000 Å thick silicon oxide film 22 as a basefilm was deposited on a Corning #7059 glass substrate 21, and anamorphous silicon film 23 was deposited further thereon at a thicknessof from 500 to 3,000 Å, for example, at a thickness of 500 Å and 1,500Å. After removing hydrogen from the film by keeping the film at atemperature of 430° C. for a duration of from 0.1 to 2 hours, forexample, 0.5 hour, a nickel film was deposited thereon by sputtering ata thickness of from 100 to 1,000 Å, for example, 500 Å. A nickelsilicide film can be used in the place of the nickel film. The nickelfilm thus obtained was etched to form patterns 24a, 24b, and 24c asshown in FIG. 2(A).

Then, the structure was heated in the temperature range of from 450° to580° C. for a duration of from 1 to 10 minutes to allow the nickel films24a to 24c to undergo reaction with the amorphous film 23 to form thincrystalline silicon regions 25a, 25b, and 25c as shown in FIG. 2(B).

The nickel film and the nickel silicide film thus obtained from thenickel film through the reaction were subjected to etching usinghydrochloric acid in a concentration of from 5 to 30%. No influence wasfound on crystalline silicon regions 25a to 25c which have been formedby the reaction between amorphous silicon and nickel (silicide) by thistreatment. Thus was obtained a structure shown in FIG. 2(C).

The resulting structure was annealed under a nitrogen atmosphere in anannealing furnace whose temperature was kept in a range of from 450° to580° C., for example, 550° C., for a duration of 4 hours. FIG. 2(D)provides an intermediate state during the annealing process, in whichthe crystallization is observed to proceed from the previously formedcrystalline silicon regions 25a to 25b, in such a manner that thecrystalline silicon regions 26a, 26b, and 26c are observed to extendinto the amorphous region 23.

A crystalline silicon film 27 was finally obtained by crystallizing theentire amorphous silicon film. In contrast to the case of Example 1 inwhich the crystal growth proceeds perpendicularly from the surface tothe substrate side, the crystal in the present example growstransversely from the patterned nickel. For instance, the crystalstructure of the crystalline silicon regions 26a to 26c as shown in FIG.2(D) is similar to that of a single crystal silicon. Accordingly, thestructure can be suitably applied to semiconductor devices such as TFTsbecause the formation of a potential barrier in these crystallinesilicon regions along the transverse direction is relatively rare.However, at portions in which the crystalline silicon regions 26a and26b collide with each other, for example, crystals are greatly damagedand are thus not suitable for the application.

FIG. 5 shows the relation between the crystallization rate and thetemperature of crystallization. It has been found that thecrystallization proceeds faster with increasing thickness of the siliconfilm.

EXAMPLE 3

The present example relates to a process for fabricating a silicon filmhaving an improved crystallinity by irradiating a laser beam to thesilicon film after once crystallizing it by heating. Furthermore, thepresent example provides a process for fabricating a TFT using the thuscrystallized silicon film.

FIG. 6 shows the cross section view of the step-sequential structuresobtained in the present process. Referring to FIG. 6, a 2,000 Å thicksilicon oxide film 602 as a base film was deposited on a Corning #7059glass substrate 601, and an intrinsic (I type) amorphous silicon filmwas deposited further thereon at a thickness of from 100 to 1,500 Å, forexample, at a thickness of 800 Å in this case. A nickel film, i.e., acatalytic material for accelerating the crystallization of the amorphoussilicon, was deposited selectively thereon by a process similar to thatdescribed in Example 2 (refer to FIG. 2(A)). The resulting structure wasthen heated in the temperature range of from 450° to 580° C. for aduration of from 1 to 10 minutes to allow the nickel film to react withthe amorphous silicon film, thereby obtaining a thin crystalline siliconfilm. The resulting structure is shown in FIG. 2(B).

The nickel film and the nickel silicide film structure obtained from thenickel film through the reaction were subjected to etching usinghydrochloric acid in a concentration of from 5 to 30%. No influence bythis treatment was found on crystalline silicon which has been formed bythe reaction between amorphous silicon and nickel (silicide). Thus wasobtained a structure shown in FIG. 2(C).

A further annealing at 550° C. for 12 hours under a nitrogen atmosphereat atmospheric pressure provides a crystalline silicon film 603 coveringthe entire surface of the structure.

Then, a KrF excimer laser was operated to irradiate a laser beam at awavelength of 248 nm and at a pulse width of 20 nsec to the surface ofthe resulting crystalline silicon film to further accelerate thecrystallization thereof. The laser beam was irradiated at an outputenergy density of from 200 to 400 mJ/cm², for instance 250 mJ/cm², for 2shots During the laser beam irradiation, the substrate was maintained ata temperature of 300° C. by heating to fully enhance the effect of laserbeam irradiation. In general, the substrate is preferably heated in thetemperature range of from 200° to 450° C. The present step isillustrated in FIG. 6(A).

Usable laser light other than that of the KrF excimer laser aboveinclude those emitted from a XeCl excimer laser operating at awavelength of 308 nm and an ArF excimer laser operating at a wavelengthof 193 nm. Otherwise, an intense light may be irradiated in the place ofa laser light. In particular, the application of RTA (rapid thermalannealing) which comprises irradiating an infrared light is effectivebecause it can selectively heat the silicon film in a short period oftime.

Thus, a silicon film having a favorable crystallinity can be obtained byemploying any of the aforementioned methods. The crystallized siliconfilm 603 obtained as a result of thermal annealing was found to changeinto a silicon film having a further improved crystallinity.Furthermore, observation by transmission electron microscope revealedthat relatively large grains of oriented crystallites constitute thelaser-irradiated film.

The silicon film thus obtained upon the completion of crystallizationwas patterned into squares 10 to 1,000 μm in edge length to obtainisland-like silicon film 603' as the active layer of the TFT, as shownin FIG. 8(B).

A silicon oxide film 804 which functions as a gate insulator film wasformed. Here, the silicon film was exposed to an oxidizing atmosphere inthe temperature range of from 500° to 750° C., preferably in thetemperature range of from 550° to 650° C., to form a silicon oxide film604 which functions as a gate insulator film on the surface of thesilicon region. In this heat treatment step, the oxidation reaction canbe more enhanced by incorporating water vapor, nitrous oxide, and thelike into the atmosphere. As a matter of course, the silicon oxide filmcan be formed by using any of the known means for vapor phase crystalgrowth, such as plasma CVD and sputtering. This step is illustrated inFIG. 6(C).

Subsequently, a polycrystalline silicon film containing from 0.01 to0.2% of phosphorus was deposited by reduced pressure CVD to a thicknessof from 3,000 to 8,000 Å, specifically 6,000 Å. A gate contact 605 wasformed thereafter by patterning the silicon film. Furthermore, animpurity (phosphorus in the present Example), to render the active layerregions (source/drain which constitute the channel) N-conductive, wasadded by ion doping (plasma doping) in a self-aligned manner using thesilicon film above as a mask. In the present Example, the ion doping wasperformed using phosphine (PH₃) as the doping gas to introducephosphorus at a dose of 1×10¹⁵ to 8×10¹⁵ cm⁻² specifically for example,5×10¹⁵ cm⁻², and at an accelerated voltage of 60 to 90 kV. Thus wereobtained N-type conductive impurity regions 606 and 607 for thesource/drain regions.

Laser was then irradiated for annealing. Though a KrF excimer laseroperated at a wavelength of 248 nm and a pulse width of 20 nsec was usedin the present Example, other lasers can be used as well. The laserlight was irradiated at an energy density of 200 to 400 mJ/cm², forexample 250 mJ/cm² and from 2 to 10 shots, for example 2 shots, persite. The effect of laser annealing can be further enhanced by heatingthe substrate in the temperature range of from 200° to 450° C. duringthe irradiation of laser light. This is illustrated in FIG. 6(D).

Otherwise, this step can be carried out by a so-called RTA (rapidthermal annealing) process, i.e., lamp annealing using near infraredlight. Since near infrared light can be more readily absorbed by acrystallized silicon than by amorphous silicon, an effective annealingwell comparable to thermal annealing at temperatures not lower than1,000° C. can be effected. More advantageously, near infrared light isless absorbed by glass substrates. The fact is that a far infrared lightis readily absorbed, but a light in the visible to near infrared region,i.e., a light in the wavelength region of 0.5 to 4 μm, is hardlyabsorbed by a glass substrate. Accordingly, the annealing can becompleted within a shorter period of time, and yet, without heating thesubstrate to a high temperature. It can be seen that this method is mostsuitable for a step in which the shrinking of the glass substrate isunfavorable.

A 6,000 Å thick silicon oxide film 608 was deposited as an interlayerinsulator by plasma CVD. A polyimide film can be used in the place ofsilicon oxide. Contact holes were perforated to form contacts withconnection 609 and 610 using a metallic material, for example, amultilayered film of titanium nitride and aluminum. Finally, annealingwas performed at 350° C. for a duration of 30 minutes under a pressureof 1 atmosphere to obtain a complete TFT structure as shown in FIG.6(E).

As described in the present Example, an amorphous silicon film can bemore favorably crystallized than in the case of simply heating byintroducing nickel as a catalytic element for the crystallization, andyet, the crystallinity of the thus crystallized silicon film can befurther ameliorated by irradiating a laser light. In this manner, acrystalline silicon having particularly high crystallinity can beobtained. The use of the resulting crystalline silicon film of goodcrystallinity provides a high performance TFT.

More specifically, an N-channel TFT obtained through the same processsteps as in the process of the present Example except for not employingthe crystallization step described in Example 2 yields a field-effectmobility of from 50 to 90 cm² /Vs, and a threshold voltage of from 3 to8 V. These values are in clear contrast to a mobility of from 150 to 200cm² /Vs and a threshold voltage of from 0.5 to 1.5 V obtained for theN-channel TFT fabricated in accordance with the present Example. Themobility is considerably increased, and the fluctuation in the thresholdvoltage is greatly reduced.

Previously, the aforementioned TFT characteristics of such a high levelwere obtained from amorphous silicon film only by laser crystallization.However, the silicon films obtained by a prior art laser crystallizationyielded fluctuation in the characteristics. Furthermore, thecrystallization process required an irradiation of a laser light at anenergy density of 350 mJ/cm² or higher at a temperature of 400° C. orhigher, and it was therefore not applicable to mass production. Incontrast to the conventional processes, the process for fabricating aTFT according to the present Example can be performed at a lowersubstrate temperature and at a lower energy density than the values forthe conventional processes. Accordingly, the process according to thepresent invention is suitable for mass production. Furthermore, thequality of the devices obtained by the present process is as uniform asthe one for the devices obtained by a conventional solid phase growthcrystallization using thermal annealing. Accordingly, TFTs of uniformquality can be obtained stably.

In the foregoing Examples 1 and 2, the crystallization was found tooccur insufficiently when the nickel concentration was low. However, theprocess according to the present Example employs laser irradiation tocompensate for an insufficient crystallization. Accordingly, TFTs withsatisfactorily high quality can be obtained even when the nickelconcentration is low. This signifies that devices containing nickel at astill lower concentration can be implemented, and that devices havingexcellent electric stability and reliability can be obtained.

EXAMPLE 4

The present example relates to a process for introducing a catalyticelement into the amorphous film by coating the upper surface of theamorphous silicon film with a solution containing a catalytic elementwhich accelerates the crystallization of the amorphous silicon film.

The present invention also provides a process for fabricating acrystalline silicon film containing a catalytic element at a lowconcentration by selectively introducing a catalytic element into anamorphous silicon film, and then allowing crystal growth to proceedtherefrom to the portions into which catalytic elements were notintroduced.

FIG. 7 shows schematically the step-sequential fabrication processaccording to the present invention. A 1,000 Å thick amorphous siliconfilm 705 was deposited on a 10×10-cm square Corning #7059 glasssubstrate 701 by plasma CVD, and a silicon oxide film 704 was depositedfurther thereon to a thickness of 1,200 Å as a mask. A silicon oxidefilm 704 as thin as 500 Å in thickness can be used without any problem,and the film can be made even thinner by using a denser film.

The resulting silicon oxide film 704 was patterned as desired by anordinary photolithographic patterning. Then, a thin silicon oxide film703 was deposited in an oxygen atmosphere by irradiating ultraviolet(UV) light. More specifically, the silicon oxide film 703 was fabricatedby irradiating the UV light for 5 minutes. The silicon oxide film 703 isbelieved to have a thickness of from 20 to 50 Å. Thus was obtained astructure as shown in FIG. 7(A).

The silicon oxide film above is formed for improving wettability of thepattern with the solution to be applied hereinafter. A favorablewettability is sometimes assured by the hydrophilic nature of thesilicon oxide mask, but this is a rare case because this happens onlywhen the pattern size matches with the solution. Accordingly, it issafer to use a silicon oxide film 703 to assure good wettability.

Then, a 5-ml portion of an acetate solution containing 100 ppm by weightof nickel was dropped on the surface of a 10×10-cm² square substrate. Aspinner 707 was operated for 10 seconds at a rate of 50 rpm to form auniform aqueous film on the entire surface of the substrate. The spinner707 was operated for an additional 60 seconds at a rate of 2,000 rpm toeffect spin drying after maintaining the substrate for 5 minutes. Thesubstrate may be subjected to rotation at a rate of from 0 to 150 rpm ona spinner. This step is illustrated in FIG. 7(B).

The resulting structure was heated at a temperature range of from 450°to 580° C. for a duration of from 1 to 10 minutes to form an extremelythin nickel silicide film on the surface of the amorphous silicon film705. A crystalline silicon was found to form at the phase boundarybetween the amorphous silicon film 705 and the nickel silicide film, aswell as in the vicinity 710 thereof in this step. The surface of theamorphous silicon film 705 was etched with 5% hydrochloric acid toremove the nickel silicide film.

Subsequently, the resulting structure was subjected to heat treatment at550° C. under a nitrogen atmosphere for a duration of 4 hours. In thismanner, crystallization is allowed to occur from the region 709 intowhich nickel was introduced to the region 710 into which nickel was notintroduced along the transverse direction as indicated with an arrow708. As a matter of course, crystallization also occurs in the region709 into which nickel was directly introduced.

In FIG. 7(C), crystallization occurred in the region 709 into whichnickel was directly introduced, and it proceeded transversely over theregion 711.

The nickel concentration can be controlled by changing the nickelconcentration of the solution to be applied. In the present invention,the nickel concentration in the solution was adjusted to 100 ppm.However, it is confirmed that crystallization also occurs even when theconcentration is decreased to 10 ppm. Crystallization occurs in the samemanner using a solution containing nickel at a concentration of 10 ppm.In this case, the nickel concentration in the region 711 as shown inFIG. 7 can be further lowered by a digit. However, the use of a solutioncontaining nickel at too low a concentration shortens the distance ofcrystal growth along the transverse direction indicated by the arrow708, and is therefore not desired.

The crystalline silicon film thus obtained can be used as it is in theactive layer of a TFT. In particular, the use of a region 711 to form anactive layer is useful because this region contains the catalyticelement at a low concentration.

It is also effective to further improve the crystallinity of thecrystalline silicon film obtained above by irradiating a laser beam oran intense light equivalent thereto in the same manner as in theforegoing Example 3.

An acetate solution was used in the present example as a solutioncontaining the catalytic element. However, other usable solutionsinclude an aqueous solution selected from a wide variety, and a solutioncontaining an organic solvent. The catalytic element need not necessarybe included as a compound, and it may be simply dispersed in thesolution.

The solvent for the catalytic element can be selected from the groupconsisting of polar solvents, i.e., water, alcohol, acids, and ammoniawater.

When nickel is used as the catalytic element, nickel is incorporatedinto a polar solvent in the form of a nickel compound. The nickelcompound is selected from, representatively, the group consisting ofnickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickelchloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate,nickel acetylacetonate, nickel 4-cyclohexylbutyrate, nickel oxide, andnickel hydroxide.

The solvent may be selected from a non-polar solvent selected from thegroup consisting of benzene, toluene, xylene, carbon tetrachloride,chloroform, and ether.

In this case, nickel is involved in the solution in the form of a nickelcompound, which is selected from the group consisting of nickelacetylacetonate, and nickel 2-ethylhexanate.

It is also effective to add a surfactant into the solution containing acatalytic element. A surfactant increases the adhesiveness of thesolution to the surface of the silicon oxide film, and controls theadsorptivity. The surfactant may be applied previously to the surface tobe coated. If elemental nickel is used as the catalytic element, it mustbe previously dissolved into an acid to obtain a solution thereof.

Instead of using a solution containing nickel completely dissolved intothe solution, an emulsion, i.e., a material comprising a dispersingmedium uniformly dispersed therein a powder of metallic nickel or of anickel compound, can be used.

The same applies in other cases using a material other than nickel asthe catalytic element.

A solution containing a non-polar solvent, i.e., a toluene solution ofnickel 2-ethylhexanate, can be directly applied to the surface of anamorphous silicon film. In this case, it is effective to use a materialsuch as an adhesive generally employed in forming a resist. However, theuse of the adhesive in an excess amount reversely interferes thetransfer of the catalytic element into amorphous silicon.

The catalytic element is incorporated into the solution approximately inan amount as nickel of, though depending on the type of the solution,from 1 to 200 ppm by weight, and preferably, from 1 to 50 ppm by weight.This range of addition is determined by taking the nickel concentrationof the crystallized film and the resistance against hydrofluoric acidinto consideration.

EXAMPLE 5

In the present example, a nickel film is patterned into islands on aglass substrate made of Corning 7059. Using this film as a startingmaterial, an amorphous silicon film is crystallized. Using the obtainedcrystalline silicon film, TFTs are fabricated. This process is describedbelow. Two methods are available to pattern the nickel film intoislands. In one method, the nickel film is formed under the amorphoussilicon film, as shown in FIG. 9(A-1). In the other, the nickel film isformed on the amorphous film, as shown in FIG. 9(A-2). In the lattermethod, after nickel is deposited over the whole surface of theamorphous silicon film, the nickel film is selectively etched. Thenickel slightly reacts with the amorphous silicon, thus producing nickelsilicide. If this silicide is left, a silicon film of high crystallinitywhat the present invention is intended to provide will not be obtained.Therefore, it is necessary to sufficiently remove the nickel silicidewith hydrochloric acid or hydrofluoric acid. Consequently, the thicknessof the amorphous silicon is reduced compared with the thickness obtainedat the beginning.

The former method does not present such a problem. It is necessary,however, to etch away the nickel film completely except for the islands.To suppress the effects of the remaining nickel, the substrate istreated with oxygen plasma, ozone, or the like to oxidize the nickelexcept for the islands.

In either method, a 2000 Å-thick silicon oxide film 101B forming a baselayer was formed on a substrate 101A made of Corning 7059 by plasma CVD.The amorphous silicon film, indicated by numeral 101, had a thickness of200 to 3000 Å, preferably 500 to 1500 Å, and was fabricated by plasmaCVD or low-pressure CVD. The amorphous silicon film was annealed at 350°to 450° C. for 0.1 to 2 hours to release hydrogen atoms. When thehydrogen concentration of the film was less than 5 atomic %,crystallization was easily conducted.

In the method shown in FIG. 9(A-1), before formation of the amorphoussilicon film 101, nickel was sputtered to a thickness of 50 to 1000 Å,preferably 100 to 500 Å. This nickel film was photolithographicallypatterned to form islands 102 of nickel.

In the method shown in FIG. 9(A-2), after the formation of the amorphoussilicon film 101, nickel was sputtered to a thickness of 50 to 1000 Å,preferably 100 to 500 Å. Then, this nickel layer wasPhotolithographically patterned to form islands 102 of nickel. FIG. 8(A)is a top view of these islands 102.

Each island of nickel is a square having sides of 2 μm, and the spacingbetween the successive islands is 5 to 50 μm, e.g., 20 μm. Similaradvantages can be obtained by using nickel silicide instead of nickel.When the nickel film was formed, good results could be obtained byheating the substrate at 100° to 500° C., preferably 180° to 250° C.,because the adhesion of the nickel film to the underlying silicon oxidefilm is improved, and because silicon oxide reacts with nickel,producing nickel silicide. Similar advantages can be derived by usingsilicon nitride, silicon carbide, or silicon instead of silicon oxide.

Then, the laminate was annealed at 450° to 650° C., e.g., 550° C., for 8hours in a nitrogen ambient. FIG. 9(B) shows an intermediate state. InFIG. 9(A), islands of nickel film located at two ends grow as nickelsilicide 103A toward the center. Those portions 103 through which thenickel has passed are made of crystalline silicon. Finally, as shown inFIG. 9(C), the two growing nickel crystal portions meet, leaving behindthe nickel silicide 103A in the center. Thus, the crystallizationprocess ends.

FIG. 8(B) is a top view of the laminate under this condition. The nickelsilicide 103A shown in FIG. 9(C) forms grain boundaries 104. If theanneal is continued, nickel atoms move along the grain boundaries 104and are collected in a region 105 located among the islands of nickel.In this stage, the original shape of the islands is not retained.

A crystalline silicon can be obtained by the steps described thus far.It is not desired that nickel atoms diffuse from the produced nickelsilicide 103A into the semiconductor film. Therefore, it is desired toetch the nickel film with hydrofluoric acid or hydrochloric acid.Neither hydrofluoric acid nor hydrochloric acid affects the siliconfilm. The laminate having the etched nickel film is shown in FIG. 9(D).The grain boundaries form grooves 104A. It is not desired to formsemiconductor regions, or active layers, of TFTs on opposite sides ofeach groove. An example of arrangement of the TFT is shown in FIG. 8(C),where semiconductor regions 108 are arranged so as not to intersect thegrain boundaries 104. On the other hand, gate interconnects 107 mayintersect the grain boundaries 104.

Examples of method of fabricating TFTs using the crystalline siliconobtained by the steps described thus far are shown in FIGS. 10, (A)-(C),and 11, (A)-(C). In FIG. 10(A), a central portion X indicates a locationwhere the groove 104A existed. The semiconductor regions of the TFTs arearranged so as not to intersect this central portion X. Morespecifically, the crystalline silicon film 103 obtained by the steps ofFIG. 9(A-1) to (D), was patterned to form island-shaped semiconductorregions 111a and 111b. Then, a silicon oxide film 112 acting as agate-insulating film was formed by RF plasma CVD, ECR plasma CVD,sputtering, or other method.

Thereafter, a polysilicon film having a thickness of 3000 to 6000 Å anddoped with phosphorus atoms at a concentration of 1×10²⁰ to 5×10²⁰ cm⁻³was formed by LPCVD. This film was photolithographically patterned toform gate electrodes 113a and 113b (FIG. 10(A)).

Then, an impurity was implanted by plasma doping. In the case of anN-type semiconductor, phosphine (PH₃) was used as a dopant gas. In thecase of a P-type semiconductor, diborane (B₂ H₆) was used as a dopantgas. In the illustrated example, N-type TFTs are shown. Phosphine ionswere accelerated at 80 kV. Diborane ions were accelerated at 65 kV. Thelaminate was annealed at 550° C. for 4 hours to activate the dopant,thus forming doped regions 114a to 114d. For the activation, a methodusing optical energy such as laser annealing or flash lamp annealing canbe used (FIG. 10(B)).

Finally, silicon oxide was deposited to a thickness of 5000 Å to form aninterlayer insulator 115 in the same way as in normal fabrication ofTFTs. Contact holes were created in this insulator 115. Conductiveinterconnects and electrodes 116a to 118d were formed (FIG. 10(C)).

TFTs were fabricated by the steps described thus far. In the illustratedexample, the TFTs were of the N-channel type. The field-effect mobilityof the obtained TFTs was 40 to 60 cm² /VS for the N-channel type and 30to 50 cm² /V s for the P-channel type.

FIG. 11, (A)-(C), show fabrication of TFTs having aluminum gates. InFIG. 11(A), a central portion X indicates a location where the groove104A (FIG. 9(D)) existed. The semiconductor regions of the TFTs do notintersect this central portion X. More specifically, the crystallinesilicon film 103 obtained by the steps of FIG. 9, (A-1)-(D), waspatterned to form island-shaped semiconductor regions 121a and 121b.Then, a silicon oxide film 122 acting as a gate-insulating film wasformed by RF plasma CVD, ECR plasma CVD, sputtering, or other method.Where plasma CVD was employed, if TEOS (tetraethoxysilane) and oxygenwere used as raw material gases, good results were obtained. Aluminumcontaining 1% silicon was sputtered to form an aluminum film having athickness of 5000 Å. This aluminum film was photolithographicallypatterned to form gate interconnects and electrodes 123a and 123b.

Then, the laminate was immersed in ethylene glycol solution of 3%tartaric acid. An electrical current was passed between a cathode and ananode which consisted of platinum and the aluminum interconnects,respectively, to effect anodization. The current was increased at a rateof 2 V/min at first. When the current reached 220 V, the voltage wasmaintained constant. When the current decreased below 10 μA/m², thecurrent was made to cease As a result, anodic oxides 124a and 124bhaving a thickness of 2000 Å were formed (FIG. 11(A)).

Then, an impurity was implanted by plasma doping. In the case of anN-type semiconductor, phosphine (PH₃) was used as a dopant gas. In thecase of a P-type semiconductor, diborane (B₂ H₈) was used as a dopantgas. In the illustrated example, N-type TFTs are shown. Phosphine ionswere accelerated at 80 kV. Diborane ions were accelerated at 65 kV. Theimpurity was activated by laser annealing to form doped regions 125a to125d. For this purpose, a KrF laser emitting a wavelength of 248 nm wasused. Five shots of laser beam of an energy density of 250 to 300 mJ/cm²were illuminated (FIG. 11(B)).

Finally, silicon oxide was deposited to a thickness of 5000 Å to form aninterlayer insulator 126 in the same way as in normal fabrication ofTFTs. Contact holes were created in this insulator 126. Conductiveinterconnects and electrodes 127a to 127d were formed in the sourceregion and in the drain regions (FIG. 11(C)).

The field-effect mobility of the obtained TFTs was 60 to 120 cm² /V sfor the N-channel type and 50 to 90 cm² /V s for the P-channel type. Ashift register was built, using these TFTs. We have confirmed that thisshift register operates at 6 MHz with a drain voltage of 17 V and at 11MHz with a drain voltage of 20 V.

EXAMPLE 6

FIG. 12, (A)-(C), show a case in which TFTs having aluminum gates werefabricated in the same manner as in the scheme illustrated in FIG. 11,(A)-(C). In this example, an active layer was fabricated from amorphoussilicon. As shown in FIG. 12(A), silicon oxide was deposited as a basefilm 132 on a substrate 131. Amorphous silicon 133 having a thickness of2000 to 3000 Å was deposited on the film 132. An appropriate amount ofP- or N-type impurity may be added to the amorphous silicon film.Subsequently, islands 134A and 134B of nickel or nickel silicide werecreated as described above. The laminate was annealed at 550° C. forfour hours to crystallize the amorphous silicon film.

The crystalline silicon film obtained in this way wasphotolithographically patterned as shown in FIG. 12(B). The silicon filmis enriched with nickel in its central portion located between theislands 134A and 134B of the nickel or nickel silicide. Therefore, thepatterning step was carried out to exclude this central portion. As aresult, island-shaped silicon regions 135A and 135B were formed. Asubstantially intrinsic amorphous silicon film 136 was deposited on theisland-shaped silicon regions 135A and 135B.

Then, as shown in FIG. 12(C), a gate-insulating film 137 was fabricatedfrom silicon nitride, silicon oxide, or the like. Gate electrodes 138ware fabricated from aluminum. In the same way as in the methodillustrated in FIG. 11, (A)-(C), anodization was effected. Impurityatoms were diffused by ion implantation to form doped regions 139A and139B. Then, an interlayer insulator 140 was deposited. Contact holeswere created. Metal electrodes 141A and 141B were formed on the sourceand drain electrodes, thus completing TFTs. These TFTs are characterizedin that the semiconductor regions of the source and drain electrodes arethick compared with the thickness of the active layer and that theresistivity is small. In consequence, the resistances of the source anddrain regions are small, and the characteristics of the TFTs areimproved. Furthermore, it is easy to form contacts.

EXAMPLE 7

FIG. 13, (A)-(C), show fabrication of CMOS TFTs. As shown in FIG. 13(A),silicon oxide was deposited as a base film 152 on a substrate 151. Anamorphous silicon film 153 having a thickness of 1000 to 1500 Å wasdeposited. Islands 154 of nickel or nickel silicide were formed asdescribed above. The laminate was annealed at 550° C. During this step,a nickel silicide region 155 was grown, and the crystallizationprogressed. The anneal was conducted for 4 hours. As shown in FIG.13(B), the amorphous silicon film changed into crystalline silicon. Asthe crystallization progressed, nickel silicide regions 159A and 159Bwere urged toward the opposite ends.

The crystalline silicon film obtained in this way was patternedphotolithographically as shown in FIG. 13(B) to form an island siliconregion 156. It is to be noted that the island region is enriched withnickel at both ends. After the formation of the island silicon region, agate-insulating film 157, and gate electrodes 158A, 158B were formed.

Then, as shown in FIG. 12(C), impurity ions were diffused by ionimplantation to form an N-type doped region 160A and a P-type dopedregion 160B. For example, phosphorus was used as the N-type impurity.Phosphine (PH₃) was used as a dopant gas. Impurity ions were implantedinto the whole surface at an accelerating voltage of 60 to 110 kV. Then,the N-channel TFTs were coated with a photoresist. P-type impurity suchas boron was implanted at an accelerating voltage of 40 to 80 kV.Diborane (B₂ H₆) was used as a dopant gas.

After the ion implantation, laser light was illuminated in the same wayas in the steps illustrated in FIG. 11, (A)-(C), to activate the sourceand drain electrodes. Subsequently, an interlayer insulator 161 wasdeposited. Contact holes were created. Metal electrodes 162A, 162B, and162C were formed on the source and drain electrodes, thus completingTFTs.

EXAMPLE 8

The present example is similar to the steps of Example 7 except that alaser illumination step was carried out to improve the crystallinity ofthe crystalline silicon film further after the crystallization step inwhich the laminate was heated at 550° C. for 4 hours.

In the present example, CMOS TFTs were fabricated in the manner as shownin FIG. 14, (A)-(D). First, as shown in FIG. 14(A), silicon oxide wassputtered to a thickness of 2000 Å to form a base film 152 on asubstrate 151. An amorphous silicon film 153 having a thickness of 1000to 1500 Å was formed by plasma CVD. Islands 154 of nickel or nickelsilicide were then created.

The laminate was annealed at 550° C. for four hours within a nitrogenambient. During this step, the nickel silicide region 155 was grown,i.e., crystallization progressed. A crystalline silicon film obtained inthis way was photolithographically patterned as shown in FIG. 14(B) toform island-shaped silicon regions 156.

KrF excimer laser radiation 171 having a wavelength of 248 nm and apulse duration of 20 nsec was illuminated. Two shots of the laserradiation were illuminated. The energy of each shot was 250 mJ/cm². Theillumination energy may be set to 200 to 400 mJ/cm², taking account ofvarious conditions such as the film thickness and the substratetemperature. An XeCl laser emitting a wavelength of 308 nm or an ArFlaser emitting a wavelength of 193 nm can be used as the laser.

Furthermore, other intense light source which can produce the sameeffects as laser illumination can also be used. Especially, rapidthermal annealing (RTA) techniques exploiting infrared irradiationpermits silicon to selectively absorb infrared radiation. Hence, ananneal can be carried out efficiently. The laser illumination may beeffected prior to the patterning step.

After the above mentioned thermal annealing, a crystallized region wasformed in the silicon film 153. However, a non-crystallized region mayremain in the silicon film 153 (not shown in the figures).

The crystallinity of the crystallized region was further improved by thesubsequent laser annealing or RTA. Therefore, this region is suitable asan active region of thin film transistors. On the other hand, while thenon-single crystallized region was also convened into a polycrystallinestructure, the result of Raman spectroscopy on this region revealed thatthe crystallinity is relatively poor as compared with the previouslycrystallized region. Also, innumerable crystallites were observedthrough transmission electron microscopy in the non-crystallized regionafter the laser annealing or RTA, while relatively large crystalsuniformly oriented were observed in the previously crystallized region.This means that the non-crystallized region includes a number of grainboundaries even after the laser annealing or RTA and therefore, is notso suitable as an active region of thin film transistors.

Therefore, it is preferable to remove the non-crystallized region so asto form silicon islands to become TFTs either before or after the laserannealing or RTA.

Then, gate electrodes 158A and 158B consisting mainly of silicon wereformed. As shown in FIG. 14(C), impurity atoms were diffused by ionimplantation to form N-type doped regions and P-type doped regions 160Aand 160B, respectively. For example, phosphorus was used as an N-typeimpurity. Phosphine (PH₃) was used as a dopant gas. The impurity ionswere implanted into the whole surface at an accelerating voltage of 60to 110 kV. Subsequently, a photoresist was coated on the N-channel TFTregions. P-type impurity ions such as boron ions were implanted at anaccelerating voltage of 40 to 80 kV, using diborane (B₂ H₆) as a dopantgas.

After the ion implantation, the source and drain electrodes wereactivated by laser illumination. An interlayer insulator 161 wasdeposited, and contact holes were created. Metal electrodes 182A, 162B,and 182C were formed on the source and drain electrodes, thus completingTFTs.

In the present example, a catalytic element was introduced to promotecrystallization. In this way, a low-temperature, short crystallizationstep was used together with an annealing step using laser illumination.The crystallization step was effected at 550° C. for about four hours.In this way, a silicon film of good crystallinity could be obtained.High-performance TFTs could be derived, using such a crystalline siliconfilm.

More specifically, the N-channel TFTs obtained in Example 5 had afield-effect mobility of 40 to 60 cm² /V s for the silicon-gate type(FIG. 10, (A)-(C)) and a field-effect mobility of 60 to 120 cm² /V s forthe aluminum gate type (FIG. 11, (A)-(C)). The threshold voltage was 3to 8 V. The mobility of the N-channel TFTs obtained in the presentexample was 150 to 200 cm² /V s, and the threshold voltage was 0.5 to1.5 V. It is to be noted that the mobility was improved greatly and thatvariations in threshold voltage decreased.

These characteristics were heretofore attainable only with lasercrystallization of an amorphous silicon film. In the prior art lasercrystallization, the obtained silicon films differed widely incharacteristics. Also, temperatures higher than 400° C. were needed forcrystallization. Furthermore, illumination of a high laser energyexceeding 350 mJ/cm² was required. Hence, mass production has sufferedfrom problems. In the present example, a lower substrate temperature anda lower energy density suffice. In consequence, mass production can becarried out without difficulty. In addition, variations are comparableto variations occurring when a solid phase crystal growth method usingconventional thermal anneal is used. Hence, the obtained TFTs areuniform in characteristics.

In the present invention, if the concentration of nickel is low, thesilicon film is not crystallized sufficiently, and the characteristicsof TFTs were not good. In the present example, however, even if thecrystallinity of the silicon film is not sufficiently high, it can becompensated for by subsequent laser illumination. Therefore, if theconcentration of nickel is low, the characteristics of the TFT are notdeteriorated. Consequently, the concentration of nickel in the activelayer regions of devices can be lowered further. This further enhanceselectrical stability and reliability of the devices.

EXAMPLE 9

In the present example, a catalytic element for promotingcrystallization of amorphous silicon is added to a solution. Thissolution is applied to the amorphous silicon film. In this way, thecatalytic element is introduced in the amorphous silicon film.

Also, in the present example, the catalytic element is selectivelyintroduced. A crystal is grown from the region in which the catalyticelement has been introduced to the region in which the catalytic elementhas not been introduced. In this way, a crystalline silicon film lightlydoped with the catalytic element is obtained.

FIG. 15, (A)-(D), schematically illustrate Steps for manufacturing thepresent example. It is to be noted that like components are indicated bylike reference numerals in both FIGS. 9 and 15.

First, silicon oxide was sputtered to a thickness of 2000 Å to form abase film 101B on a glass substrate made of Corning 7059. The substratewas 10 cm square. Then, an amorphous silicon film 101 having a thicknessof 1000 Å was formed by plasma CVD.

Thereafter, a silicon oxide film 180 having a thickness of 2000 Å wasformed. Our experiment has demonstrated that if the thickness of thesilicon oxide film 180 is set to 500 Å, then no problems take place. Weconsider that if the film is dense, the film thickness can be reducedfurther.

The silicon oxide film 180 was patterned into a desired pattern bynormal photolithography techniques. Ultraviolet radiation wasilluminated for 5 minutes in an oxygen ambient to form a thin siliconoxide film 182 on the exposed surface of the amorphous silicon film 101.We think that the thickness of the silicon oxide film 182 isapproximately 20 to 50 Å (FIG. 15(A)).

This silicon oxide film is intended to improve the wettability of thesolution applied in a later step. Under this condition, 5 ml of acetatesolution was dripped onto a substrate 10 cm square. The acetate solutionwas prepared by adding 100 ppm by weight of nickel to acetate solution.At this time, the laminate was spun at 50 rpm by a spinner 84, and auniform water film 183 was formed over the whole surface of thesubstrate. This condition was maintained for 5 minutes. Then, thelaminate was spun at 2000 rpm for 60 seconds by the spinner 184 to drythe laminate. The water film may also be maintained on the spinner byrotating the laminate at 0 to 150 rpm (FIG. 15(A)).

Nickel was introduced in region 185 by the steps described above. Thelaminate was thermally treated at 300° to 500° C. to form nickelsilicide on the surfaces of the regions 185. Then, the silicon oxidefilm 180 acting as a mask was removed. The laminate was heated at 550°C. for four hours in a nitrogen ambient. In this way, the amorphoussilicon film 180 was crystallized. At this time, the crystal was grownlaterally, i.e., parallel to the substrate, from the regions 185 dopedwith nickel to regions in which nickel was not introduced. Of course,crystallization takes place in regions where nickel was directlyintroduced.

Thermal processing was performed to form a silicon nickel film on thesurfaces of the regions 185, followed by removal of the silicon oxidefilm 180. In a modified example, the laminate was heated at 550° C. forfour hours without removing the silicon oxide film 180, andcrystallization was induced. In this case, the step for creating thenickel silicide film was not required. The silicon oxide film 180 may beremoved after the crystallization step.

FIG. 15(B) shows a state in which crystallization is in progress. Inparticular, nickel which was introduced in marginal portions goes asnickel silicide 103A toward the center. The portions 10S through whichnickel has passed are crystalline silicon. If the crystallizationproceeds further, the two portions starting from the portions 185 inwhich nickel was introduced meet, as shown in FIG. 15(C), leaving behindthe nickel silicide 103A between them. Thus, the crystallization processends.

A crystalline silicon could be obtained by the steps described above. Itis not desired that nickel diffuse from the resulting nickel silicide103A into the semiconductor film. Accordingly, regions 103A were etchedaway with hydrofluoric acid or hydrochloric acid. This condition isshown in FIG. 15(D). The portions where grain boundaries existed formedgrooves 104A.

In FIG. 15(C), crystallization proceeded laterally from the regions 185through regions 188. The concentration of nickel in the regions 186 areshown in FIG. 16, which indicates the distribution of nickel in thedirection of thickness of the regions 186 of the crystalline siliconfilm which has undergone the crystallization step. The distribution wasmeasured by SIMS. It has been confirmed that the concentration of nickelin the regions 185 in which nickel was directly introduced is higherthan the concentrations whose distribution is shown in FIG. 16 by atleast one order of magnitude. Using the crystalline silicon filmobtained in this way, TFTs were fabricated by the same method as wasused in Example 5.

The crystalline silicon film derived in this way was illuminated withlaser light or other equivalent intense light to improve thecrystallinity further effectively in the same way as in Example 8. InExample 8, the concentration of nickel in the silicon film wasrelatively high and so laser illumination caused nickel to be depositedout of the silicon film. Particles of nickel having dimensions of theorder of 0.1 to 10 μm were formed in the silicon film. As a result, themorphology of the film deteriorated. In the present example, however,the concentration of nickel can be made much lower than those obtainedin Examples 5 and 6. Hence, nickel silicide was not deposited. Also, thefilm was not toughened by laser illumination.

The nickel concentration shown in FIG. 16 can be controlled bycontrolling the nickel concentration in the solution. In the presentexample, the nickel concentration in the solution was set to 100 ppm. Wehave confirmed that crystallization is possible if the concentration isset to 10 ppm. In this case, the nickel concentration (FIG. 16) in theregions 186 shown in FIG. 15, (A)-(D), can be reduced further by oneorder of magnitude. However, if the nickel concentration of the solutionis reduced, the lateral crystal growth distance shortens.

The silicon film which was crystallized as described thus far can bedirectly used as an active layer of each TFT. Especially, formation ofthe active layer of each TFT using regions 186 is quite useful in thatthe concentration of the catalytic element is low.

In the present example, acetate solution is used as a solutioncontaining a catalytic element. This solution can be selected fromvarious water solutions and organic solvents and solutions. The form ofthe catalytic element is not limited to compounds. The catalytic elementmay be simply dispersed in a solution.

Where nickel is used as a catalyst and contained in a polar solvent suchas water, alcohol, acid, or ammonia, the nickel is introduced as anickel compound. Typical examples of the nickel compound include nickelbromide, nickel acetate, nickel oxalate, nickel carbonate, nickelchloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate,nickel acetylacetonate, 4-cyclohexyl butyric nickel, nickel oxide, andnickel hydroxide.

The solvent can be selected from non-polar solvents, i.e., benzene,toluene, xylene, carbon tetrachloride, chloroform, and ether.

In this case, the nickel is introduced in the form of a nickel compound.Typical examples of the nickel compound are nickel acetylacetonate, and2-ethylhexylic nickel.

Also, it is advantageous to add a surfactant to the solution containinga catalytic element. This improves the adhesion to the applied surfaceand controls the adsorptivity. This surfactant may be previously appliedto the surface. Where simple nickel is used as the catalytic element, itis necessary to dissolve it in an acid, thus producing a solution.

In the example described above, a solution in which nickel, or acatalytic element, is fully dissolved is used. It is not alwaysnecessary that nickel be fully dissolved. In this case, a material suchas emulsion may be used which comprises a medium in which powder ofsimple nickel or a nickel compound is uniformly dispersed. Also, asolution used for formation of an oxide film may be employed. OCD (Ohkadiffusion source) produced by TOKYO OHKA KOGYO CO., LTD. can be used asthe solution. Where this OCD solution is used, it is applied to asurface to be coated, and then it is baked at about 200° C. In this way,a silicon oxide film can be easily formed. Furthermore, any impurity canbe utilized, because an impurity can be added at will.

These principles apply where materials other than nickel are used ascatalytic elements. Where a non-polar solvent such as toluene solutionof 2-ethylhexylic nickel is used, it can be directly applied to thesurface of an amorphous silicon film. In this case, it is advantageousto previously apply a material such as an intimate contact agent usedfor resist application. However, if the amount of application is toolarge, the introduction of the catalytic element in the amorphoussilicon will be hindered.

The amount of the catalytic element contained in the solution depends onthe kind of the solution. Roughly, the ratio of the weight of nickel tothe weight of solution is 200 to 1 ppm, preferably 50 to 1 ppm. Thisrange has been determined, taking account of the concentration of nickelin the film after completion of crystallization and the resistance tohydrofluoric acid.

In the present example, a solution containing a catalytic element isapplied to the top surface of an amorphous silicon film. Before theformation of the amorphous silicon film, a solution containing thecatalytic element may be applied to the base film.

EXAMPLE 10

A method of obtaining a crystalline silicon film by forming a nickelfilm on a substrate of Corning 7059 glass and crystallizing an amorphoussilicon film with this nickel film as a catalyst will now be describedwith reference to FIG. 17. On a substrate 201, a base silicon oxide film202 of thickness 2000 Å was formed by the plasma CVD method. Next, anickel film 203 of thickness less than 1000 Å, for example 50 Å, wasdeposited by sputtering. The nickel film of thickness less than 100 Åwas of a form which would be more accurately described as particles, orclusters of pluralities of particles joined together, than as a film.Good results were obtained when the substrate was heated to 100° to 500°C., preferably 180° to 250° C., for the formation of the nickel film.This is because the adhesion between the silicon oxide base film and thenickel film is improved. A nickel silicide could have been used in placeof the nickel. (FIG. 17(A))

After that, an amorphous silicon film 204 of thickness 500 to 3000 Å,for example 1500 Å, was deposited by plasma CVD, and hydrogen purgingwas carried out in a nitrogen atmosphere at 430° C. for 0.1 to 2 hours,for example 0.5 hours. (FIG. 17(B))

Next, this was annealed in a nitrogen atmosphere in an annealing furnaceat 450° to 580° C., for example 550° C., for 8 hours. FIG. 17(C)illustrates the state during this annealing in which crystallizationprogresses as nickel diffuses from the previously formed nickel film(particles, clusters) and crystalline silicon regions 205 grow andspread throughout the amorphous region 204A.

After the crystallization finished, a temperature of 400° to 600° C.,for example 550° C., was maintained, trichloroethylene (C₂ HCl₃) wasdiluted with hydrogen or oxygen to 1 to 10%, for example 10%, andintroduced into the annealing furnace, and annealing was carried out for0.1 to 2 hours, for example 1 hour. When the specimen thus chlorinationtreated was analyzed by secondary ion material spectrometry (SIMS), theconcentration of nickel in the silicon film was 0.01 atomic %. Theconcentration of nickel in a specimen on which-chlorination treatmentwas not performed was as much as 5 atomic %.

EXAMPLE 11

A eleventh preferred embodiment is illustrated in FIG. 18. A basesilicon oxide film 202 of thickness 2000 Å was formed on a Corning 7059glass substrate 201 by plasma CVD. Next, an amorphous silicon film 204of thickness 500 to 3000 Å, for example 1500 Å, was deposited by plasmaCVD, and hydrogen purging was carried out in an atmosphere of nitrogenat 430° C. for 0.1 to 2 hours, for example 0.5 hours.

After that, a nickel film 203 of thickness less than 1000 Å, for example80 Å, was deposited by sputtering. The nickel film of thickness lessthan 100 Å was of a form which would be more accurately described asparticles, or clusters of pluralities of particles joined together, thanas a film. For formation of the nickel film, good results were obtainedwhen the substrate was heated to 100° to 500° C., and preferably 180° to250° C. This is because the adhesion between the silicon oxide base filmand the nickel film improves. A nickel silicide could have been used inplace of the nickel. (FIG. 18(A))

Next, this was annealed in a nitrogen atmosphere in an annealing furnaceat 450° to 580° C., for example 550° C., for 4 hours. FIG. 18(B)illustrates the way in which, during this annealing, as nickel diffusesfrom the previously formed nickel film (particles, clusters),crystallization progresses, and crystalline silicon regions 205 grow andspread throughout the amorphous region 204A.

After the crystallization finished, a temperature of 400° to 600° C.,for example 580° C., was maintained, trichloroethylene (C₂ HCl₃) wasdiluted with hydrogen or oxygen to 1 to 10%, for example 5%, andintroduced into the annealing furnace, and annealing was carried out for0.1 to 2 hours, for example 0.5 hours.

EXAMPLE 12

A twelfth preferred embodiment is illustrated in FIG. 19. A base siliconoxide film 232 of thickness 2000 Å was formed on a Corning 7059 glasssubstrate 231 by plasma CVD. Next, a nickel film 233 of thickness lessthan 1000 Å, for example 80 Å, was deposited by sputtering. (FIG. 19(A))

The whole surface was then coated with a photoresist, and using acommonly known photolithography method a resist pattern 234 was formed.(FIG. 19(B))

This was then immersed in a suitable etchant, for example 5 to 30%hydrochloric acid solution, and the exposed parts of the nickel filmwere removed. The film can be removed in the same way in cases wherenickel silicide was used. (FIG. 19(C))

The photoresist was then removed by a commonly known method, and anickel film pattern 235 was formed. (FIG. 19(D))

After that, an amorphous silicon film was deposited by plasma CVD to athickness of 500 to 300 Å, for example 1500 Å, and hydrogen purging wascarried out in a nitrogen atmosphere at 430° C. for 0.1 to 2 hours, forexample 0.5 hours. Next, this was annealed in a nitrogen atmosphere inan annealing furnace at 450° to 580° C., for example 550° C., for 4hours. FIG. 19(E) illustrates the way in which, during this annealing,as nickel diffuses from the previously formed nickel film pattern,crystallization progresses, and crystalline silicon regions 236 grow andspread throughout the amorphous region 237.

After the crystallization finished, a temperature of 400° to 600° C.,for example 580° C., was maintained, hydrogen chloride (HCL) was dilutedwith hydrogen or oxygen to 1 to 10%, for example 10%, and introducedinto the annealing furnace, and annealing was carried out for 0.1 to 2hours, for example 0.5 hours. When the specimen thus chlorinationtreated was analyzed by secondary ion material spectrometry (SIMS), theconcentration of nickel in the silicon film was 5 to 10 PPM. Theconcentration of nickel in a specimen on which chlorination treatmentwas not performed was as much as 1 atomic %.

EXAMPLE 13

A thirteenth preferred embodiment is illustrated in FIG. 20. A basesilicon oxide film 242 of thickness 2000 Å was formed on a Corning 7059glass substrate 241 by plasma CVD. Next, an amorphous silicon film 243of thickness 500 to 3000 Å, for example 1500 Å, was deposited by plasmaCVD, and then a nickel film 244 of thickness less than 1000 Å, forexample 80 Å, was deposited by sputtering. (FIG. 20(A))

The whole surface was coated with a photoresist, and using a commonlyknown photolithography method a resist pattern 245 was formed. (FIG.20(B))

This was then immersed in a suitable etchant, for example 5 to 30%hydrochloric acid solution, and the exposed parts of the nickel filmwere thereby removed. (FIG. 20(C))

The photoresist was then removed by a commonly known method, and anickel film pattern 246 was formed. (FIG. 20(D))

After that, hydrogen purging was carried out in a nitrogen atmosphere at430° C. for 0.1 to 2 hours, for example 0.5 hours. Next, this wasannealed in a nitrogen atmosphere in an annealing furnace at 450° to580° C., for example 550° C., for 4 hours. FIG. 20(E) illustrates theway in which, during this annealing, as nickel diffuses from thepreviously formed nickel film pattern, crystallization progresses, andcrystalline silicon regions 247 grow and spread throughout the amorphousregion 248.

After the crystallization finished, a temperature of 400° to 600° C.,for example 580° C., was maintained, trichloroethylene (C₂ HCl₃) wasdiluted with hydrogen or oxygen to 1 to 10%, for example 5%, andintroduced into the annealing furnace, and annealing was carried out for0.1 to 2 hours, for example 0.5 hours. When the specimen thuschlorination treated was analyzed by secondary ion material spectrometry(SIMS), the concentration of nickel in the silicon film was 5 to 10 PPM.The concentration of nickel in a specimen on which the abovechlorination treatment was not performed was as much as 0.1 to 1 atomic%.

EXAMPLE 14

A fourteenth preferred embodiment is illustrated in FIG. 21. A basesilicon oxide film 252 of thickness 2000 Å was formed on a Corning 7059glass substrate 251 by plasma CVD. The whole surface was then coatedwith a photoresist, and using a commonly known photolithography method aresist pattern 253 was formed. (FIG. 21(A))

Next, a nickel film 254 was deposited to a thickness of 80 bysputtering. (FIG. 21(B))

The photoresist was then removed by a commonly known method and thenickel film adhered to the top of the resist was also removed at thesame time, thereby producing a nickel film pattern 255. (FIG. 21(C))

After that, an amorphous silicon film was deposited to a thickness of1000 Å by plasma CVD. Hydrogen purging was not carried out. Next, thiswas annealed in a nitrogen atmosphere in an annealing furnace at 450° to580° C., for example 550° C., for 4 hours. FIG. 21(E) illustrates theway in which, during this annealing, as nickel diffuses from thepreviously formed nickel film pattern, crystallization progresses, andcrystalline silicon regions 256 grow and spread throughout the amorphousregion 257.

After crystallization finished, a temperature of 550° C. was maintained,trichloroethylene (C₂ HCl₃) was diluted with hydrogen or oxygen to 1 to10%, for example 5%, and introduced into the annealing furnace, andannealing was carried out for 0.5 hours.

EXAMPLE 15

A fifteenth preferred embodiment is illustrated in FIG. 22. A basesilicon oxide film 262 of thickness 2000 Å was formed on a Corning 7059glass substrate 261 by plasma CVD. After that, an amorphous silicon film283 of thickness 500 Å was deposited by plasma CVD. Hydrogen purging wasnot carried out. The whole surface was then coated with a photoresist,and using a commonly known photolithography method a resist pattern 264was formed. (FIG. 22(A))

Next, a nickel film 285 was deposited to a thickness of about 100 Å byelectron beam vaporization. (FIG. 22(B))

The photoresist was then removed by a commonly known method and thenickel film adhered to the top of the resist was also removed at thesame time, thereby producing a nickel film pattern 266. (FIG. 22(C))

Next, this was annealed in a nitrogen atmosphere in an annealing furnaceat 550° C. for 4 hours. FIG. 22(E) illustrates the way in which, duringthis annealing, as nickel diffuses from the previously formed nickelfilm pattern, crystallization progresses, and crystalline siliconregions 287 grow and spread throughout the amorphous region 268.

After crystallization finished, a temperature of 500° C. was maintained,hydrogen chloride (HCL) was diluted with hydrogen or oxygen to 1 to 10%,for example 1%, and introduced into the annealing furnace, and annealingwas carried out for 0.5 hours.

EXAMPLE 16

A sixteenth preferred embodiment is characterized in that a crystallinesilicon film having good crystalline properties is obtained by a processin which a catalyst element which promotes the crystallization of anamorphous silicon film is introduced into an amorphous silicon film andcrystallization is brought about by heating and a process in which afterthis first process the crystallinity is further raised by irradiationwith laser light.

The manufacturing processes of this seventh preferred embodiment will bedescribed below with reference to FIG. 23. First, on a substrate 201, abase silicon oxide film 202 of thickness 2000 Å was formed by plasmaCVD. Next, a nickel film 203 of thickness less than 1000 Å, for example50 Å, was deposited by sputtering. The nickel film of thickness lessthan 100 Å was of a form which would be more accurately described asparticles, or clusters of pluralities of particles joined together, thanas a film. Good results were obtained when the substrate was heated to100° to 500° C., preferably 180° to 250° C., for the formation of thenickel film. This is because the adhesion between the silicon oxide basefilm and the nickel film improves. A nickel silicide could have beenused instead of nickel. (FIG. 23(A))

After that, hydrogen purging was carried out in an atmosphere ofnitrogen at 430° C. for 0.1 to 2 hours, for example 0.5 hours. (FIG.23(B))

Next, this was annealed in a nitrogen atmosphere in an annealing furnaceat 450° to 580° C., for example 550° C., for 8 hours. FIG. 23(C)illustrates the way in which, during this annealing, as nickel diffusesfrom the previously formed nickel film (particles, clusters),crystallization progresses, and crystalline silicon regions 205 grow andspread within the amorphous region 204A.

After the heat annealing was finished, annealing by irradiation withlaser light 271 was carried out. For the laser light, a KrF excimerlaser (wavelength 248 nm, pulse width 20 nsec) was used. The laser lightirradiation was performed for 2 shots at an energy density of 250mJ/cm². The substrate was heated to 300° C. for the laser lightirradiation. This is to increase the effect of the laser lightirradiation.

For the laser light, an XeCl excimer laser (wavelength 308 nm), or anArF excimer laser (wavelength 193 nm) or the like can alternatively beused. Alternatively, a method based on irradiation not with laser lightbut rather with strong light may be employed. In particular, RTA (RapidThermal Annealing), which is based on short-duration irradiation withinfrared light, has the advantage that it is possible to selectivelyheat silicon film in a short period of time.

After the completion of annealing by irradiation with laser light, atemperature of 400° C. to 600° C., for example 550°, was maintained,trichloroethylene (C₂ HCl₃) was diluted with hydrogen or oxygen to 1 to10%, for example 10%, and introduced into the annealing furnace, andannealing was carried out for 0.1 to 2 hours, for example 1 hour. Inthis way it was possible to obtain a crystalline silicon film.

When a specimen thus chlorination treated was analyzed by secondary ionmaterial spectrometry (SIMS), the concentration of nickel in the siliconfilm was 1×10¹⁸ cm⁻³. The concentration of nickel in a specimen on whichchlorination treatment was not performed was as much as 1×10¹⁹ cm⁻³.

It is possible to obtain a silicon film with good crystalline propertiesin the way described above. As a result of carrying out this treatment,the region 205 that had been crystallized by heat annealing became asilicon film of good crystalline quality. On the other hand, although asa result of the laser irradiation a polycrystalline film was alsoobtained in the region 204A which had not been crystallized, and achange in the quality of the film was observed, it was found by Ramanspectrometry that the crystalline quality in this region was not good.Also, examination with a transmission electron microscope revealed thatwhereas in the region 4A, which was crystallized by being irradiated bythe laser while still uncrystallized, countless small crystals had beenformed, the region 205, which was laser-irradiated after having alreadybeen crystallized, relatively large crystals with their crystals alignedin the same direction were observed.

When a silicon film 205 obtained in this way was formed into an islandshape and a TFT was made, a marked improvement in performance wasobserved. That is, whereas in an N channel type TFT made using a siliconfilm crystallized according to the first preferred embodiment describedabove the electric field effect mobility was 40 to 60 cm² /Vs and thethreshold voltage was 3 to 8 V, in an N channel type TFT manufactured byexactly the same method but made using a silicon film obtained accordingto this preferred embodiment the mobility was 150 to 200 cm² /Vs and thethreshold voltage was 0.5 to 1.5; the mobility was greatly improved, andthe variation in the threshold voltage was reduced.

Although in the past it has been possible to obtain this kind ofperformance by crystallization of amorphous silicon films by lasercrystallization alone, with conventional laser crystallization there hasbeen wide variation in the characteristics of the silicon films obtainedand there has been the problem that mass-producability has been poorbecause temperatures of over 400° C. and irradiation at high laserenergies of over 350 mJ/cm² have been necessary for the crystallization.With respect to this point, with this preferred embodiment, because boththe substrate temperature and the energy density are entirelysatisfactory at lower values than these, there were no problems relatingto mass-producability. Furthermore, because the variation incharacteristics is about the same as that encountered with solid phasegrowth crystallization by conventional thermal annealing, the TFTsobtained also had uniform characteristics.

It is noted that with this invention when the concentration of the Niwas too low the crystallization of the silicon film was not satisfactoryand the characteristics of the obtained TFTs were not good. However, inthis preferred embodiment, because even if the crystallinity of thesilicon film is unsatisfactory it can be made up for by the subsequentlaser irradiation, even when the Ni concentration was low there was nodeterioration in the characteristics of the TFTs. As a result of this,it is possible to further reduce the nickel concentration in the activelayer regions of the device and a structure extremely favorable to theelectrical stability and reliability of the device can be adopted.

EXAMPLE 17

An seventeenth preferred embodiment of the invention relates to a methodin which the nickel serving as the catalyst element is introduced in theliquid phase. The manufacturing processes of this eighth preferredembodiment will be described below with reference to FIG. 24. First, abase silicon oxide film 286 of thickness 2000 was formed on a 10cm-square Corning 7059 glass substrate 281 by plasma CVD. After that, anamorphous silicon film 283 was deposited to a thickness of 500 Å byplasma CVD. Then, a silicon oxide film of thickness 1500 Å was formedover the whole surface, and a mask pattern 284 was formed using acommonly known photolithography method. This mask pattern 284 of siliconoxide film is for selectively introducing nickel. Resist may be used,instead of silicon oxide, for the mask pattern.

Next, a thin silicon oxide film 282 was formed by ultravioletirradiation in an oxygen atmosphere. This silicon oxide film 282 wasmade by irradiation with UV light for 5 minutes in an oxygen atmosphere.The thickness of this silicon oxide film 282 is thought to be about 20to 50 Å. This silicon oxide film is formed in order to improve the leakcharacteristic of a solution that is applied in a later process. In thisstate, 5 ml (in the case of a 10 cm square substrate) of an acetatesolution made by adding 100 ppm (weight conversion) to an acetatesolution was dripped onto the specimen. Spin coating at 50 rpm wasperformed for 10 seconds on a spinner 280, and a water film 285, uniformover the whole surface of the substrate, was thereby formed. Then, afterthis state had been held for 5 minutes, spin drying at 2000 rpm for 60seconds was carried out using the spinner 280. This holding can beperformed on the spinner while the spinner is rotated at 0 to 150 rpm.(FIG. 24(A))

The state illustrated in FIG. 24(B) is thus obtained. This state is onein which the catalyst element nickel is in contact with the amorphoussilicon film 283 through the extremely thin oxide film 282.

Next, this was annealed in an annealing furnace at 550° C. for 4 hoursin a nitrogen atmosphere. At this time, nickel diffuses through theoxide film 282 into the amorphous silicon film, and crystallizationprogresses.

FIG. 24(C) illustrates the way in which, during this annealing, asnickel diffuses from the oxide film 282 portions, crystallizationprogresses, and crystalline silicon regions 287 grow and spreadthroughout the amorphous region 288.

After the crystallization finished, a temperature of 500° C. wasmaintained, hydrogen chloride (HCL) was diluted with hydrogen or oxygento 1 to 10%, for example 1%, and introduced into the annealing furnace,and annealing was carried out for 0.5 hours.

In this way, it was possible to obtain a crystalline silicon film. FIG.25 shows the results of a SIMS investigation of the nickel concentrationin a crystalline silicon film of which the crystallization process isfinished. The region of which the nickel concentration was investigatedis the region which was protected by the silicon oxide film 284 servingas a mask, and is a region into which nickel was not directlyintroduced.

Also, it has been confirmed that the concentration of nickel in theregion into which nickel was directly introduced, i.e. the region intowhich nickel diffused through the oxide film 282, is one order higherthan the concentration distribution shown in FIG. 25. Further improvingthe crystallinity of a silicon film obtained in this way by irradiationwith laser light or an equivalent strong light, as per the secondpreferred embodiment described above, is effective.

It is possible to control the nickel concentrations shown in FIG. 25 bycontrolling the nickel concentration in the solution. Although in thispreferred embodiment the nickel concentration in the solution was set at100 ppm, it has been found that crystallization is still possible whenthis concentration is set to 10 ppm. When this is done, the nickelconcentrations shown in FIG. 25 can be further reduced by one order.However, there is the problem that when the nickel concentration in thesolution is made low, the crystal growth distances becomes short.

Crystalline silicon film crystallized in the way described above can beused as it is for the active layers of TFTs. In particular, usingregions in which crystal growth has taken place from the region intowhich nickel was introduced in a direction parallel to the substrate toform the active layers of TFTs is extremely advantageous in that theircatalyst element concentration is low.

In this preferred embodiment, as the solution containing the catalystelement, acetate solution was used; however, it is possible to widenthis and use water solutions or organic solvent solutions or the like.Here the catalyst element does not have to be included as a chemicalcompound and may alternatively be included by simple dispersion.

As the solvent which is made to contain the catalyst element, any of thepolar solvents water, alcohol, acid or ammonia can alternatively beused.

In cases when nickel is used as the catalyst and this nickel is to beincluded in a polar solvent, the nickel is introduced in the form of achemical compound of nickel. As this nickel compound, nickel compoundschosen from among for example nickel bromide, nickel acetate, nickeloxalate, nickel carbonate, nickel chloride, nickel iodide, nickelnitrate, nickel sulfate, nickel formate, nickel acetylacetonate,4-cyclohexyl butyric acid nickel, nickel oxide and nickel hydroxide canbe used.

Also, as the solvent, any of the non-polar solvents benzene, toluene,xylene, carbon tetrachloride, chloroform and ether can be used.

In this case, nickel is introduced as a chemical compound of nickel. Asthis nickel compound, nickel compounds chosen from among for examplenickel acetylacetonate and 2-ethyl hexanoic acid nickel can be used.

The addition of a detergent to the solution containing the catalystelement is beneficial. This is because it raises the adhesion with thesurface that is coated and controls the adsorbency. This detergent maybe coated in advance onto the surface that is to be coated. When simplenickel is used as the catalyst element, it is necessary for the nickelto be first made into a solution by dissolving in acid.

That which is discussed above is an example in which a solution made bydissolving the catalyst element nickel completely is used; however, anemulsion-like substance in which a powder consisting of simple nickel ora compound of nickel is uniformly dispersed in a dispersion medium mayalternatively be used instead of completely dissolving the nickel.Alternatively, a solution for forming oxide films can be used. As thiskind of solution, OCD (Ohka Diffusion Source), made by Tokyo OhkaChemical Industries Co., Ltd., can be used. By coating this OCD onto anon-crystalline silicon film and baking it at about 200° C., a siliconoxide film can be formed simply. A catalyst element can then be causedto diffuse from this silicon oxide film into the non-crystalline siliconfilm.

It is noted that these points also apply to cases where a material otherthan nickel is used as the catalyst element.

Also, by using a non-polar solvent such as a toluene solution of 2-ethylhexanoic acid nickel as the solution it is possible to coat the solutiondirectly onto the surface of the amorphous silicon. In this case it isbeneficial that a material like an adhesive used in the application ofresist be applied in advance. However, because when the amount appliedis too great this can have the reverse effect of obstructing theaddition of the catalyst element to the amorphous silicon film, cautionis necessary.

The quantity of the catalyst element to be included in the solutiondepends on the type of solution, but as a general rule it is desirablethat the quantity of nickel with respect to the solution be 1 ppm to 200ppm, and preferably 1 ppm to 50 ppm (weight conversion). This is a valuedetermined in consideration of the nickel concentration and resistanceto fluoric acid of the film after crystallization is finished.

EXAMPLE 18

An example of positional relation among a crystallized region, an activelayer (channel region) of a TFT, a contact hole, and a region to which acatalytic element is added in forming the TFT according to the presentinvention is described in this EXAMPLE 18. A pixel part of anactive-matrix is described below.

FIGS. 26(A) to 26(F) show steps for manufacturing a TFT of this example.At first, as shown in FIG. 26(A), a silicon oxide base film 92 isdeposited on a substrate 91 to a thickness of 2000 Å by sputtering.Further, an amorphous silicon film 93 is deposited to a thickness of 300to 1500 Å, e.g. 800 Å, by plasma CVD. A silicon oxide film 94 of 200 to2000 Å, e.g. 300 Å, is formed and perforated to form holes 96a and 96b.Thus, the silicon oxide film 94 is patterned into a mask. Then, anextremely thin nickel or nickel compound film 95 is formed on the entiresurface by sputtering or spin coating as employed in Example 9.

Next, annealing is performed in a nitrogen atmosphere at 550° C. for 4hours. By this step, portions 97a and 97b of the silicon film 93 locatedjust under the holes 96a and 96b is changed into a silicide, and siliconregions 98a and 98b are crystallized from the portions. The endingportions have a high nickel concentration. (FIG. 26(B))

After sufficient crystallization, crystallization proceeding from theholes 96a and 96b collides with each other at a midway therebetween andstops at the midway. A region 99a having a high nickel concentrationremains at the midway. A photo anneal may be further performed in thiscondition by an excimer laser and the like as employed in Example 8.(FIG. 26(C))

Next, the crystalline silicon film thus obtained is patterned to form anisland silicon region 400 as shown in FIG. 26(D). A portion of highnickel concentration regions 97a and 99c remains in the silicon region400. Further, a silicon oxide gate insulating film 401 of 700 to 2000 Å,e.g. 1200 Å, in thickness is formed by plasma CVD. (FIG.26(D))

Thereafter, an aluminum gate electrode 402 is formed by the same meansas in Example 5. An anodic oxide 403 having a thickness of 1000 to 5000Å is formed around the gate electrode. Then, N-type impurity regions 404and 405 are formed by diffusing an impurity by ion doping method. Thegate electrode should be located so that the high nickel concentrationregions 97a and 99c be located out of the portion (the channel region)located just under the gate electrode as shown in FIG. 2B(E).

After doping, the source and drain are activated by laser lightirradiation. Further, an interlayer insulator 406 is deposited, and atransparent conductive film having a thickness of 500 to 1500 Å, e.g.800 Å, is formed by sputtering and patterned by etching to form a pixelelectrode 407. Further, contact holes are formed in the interlayerinsulator 406, and metallic electrodes 408 and 409 are formed to thesource and drain to complete the TFT.

It is desirable to form the contact holes apart from the high nickelconcentration regions. 97a and 99c. This can be realized by designingthe contact holes so that the contact holes does not overlap the holes96a and 96b for adding nickel. Otherwise, a defective contact wouldreadily be formed by the overetch of the silicon film during theformation of the contact holes because the high nickel concentrationregions are readily etched by a solution containing hydrogen fluoridegroup as compared with the silicon film containing no nickel. In thedrawing, the contact on the left side partly overlaps the high, nickelconcentration region 97a. It is desirable that at least a portion of theelectrode is in contact with a region other than the region to whichnickel is added.

As described above, this invention is revolutionary in the sense that itprovides a way of advancing the use of low temperatures and short timeperiods in the crystallization of amorphous silicon; also, because theequipment, apparatus and techniques used to achieve this are bothextremely ordinary and highly suited to mass-production, the potentialbenefits of the invention to industry are enormous.

For example, whereas in the conventional solid phase growth method,because annealing for at least 24 hours was held to be necessary, if thesubstrate treatment time for one sheet of substrate was 2 minutes thenas many as 15 annealing furnaces were necessary, with this invention,because the time required for annealing can be reduced to less than 4hours, the number of annealing furnaces can be cut down to less than 1/6of the number that were required before. The improvement in productivityand reductions in capital investment in equipment that result from thislead to reductions in substrate treatment costs and hence to reductionsin TFT prices and to consequent stimulation of fresh demand for TFTs.This invention is thus of industrial value and is worthy to receive apatent.

What is claimed is:
 1. A thin-film transistor comprising:a silicon filmcontaining at least one material of nickel, iron, cobalt, platinum andpalladium; an insulating film provided on said silicon film; and a gateelectrode provided on said insulating film; wherein concentration ofsaid material does not exceed 1×10¹⁹ atom/cm³.
 2. The transistor ofclaim 1 wherein said concentration of said material is 1×10³ atoms/cm³or more.
 3. The transistor of claim 1 wherein said silicon film contains1×10¹⁵ atoms/cm³ to 5 atomic % of hydrogen.
 4. A thin-film transistorcomprising:a source and a drain; wherein at least one of said source andsaid drain consists of a semiconductor containing at least one materialof nickel, iron, cobalt, platinum and palladium; and concentration ofsaid material does not exceed 1×10¹⁹ atoms/cm³.
 5. The transistor ofclaim 4 wherein said concentration of said material is 1×10¹⁵ atoms/cm³or more.
 6. The transistor of claim 4 wherein said silicon film contains1×10¹⁵ atoms/cm³ to 5 atomic % hydrogen.
 7. A semiconductor comprising:acrystalline silicon film; wherein said silicon film contains at leastone material of nickel, iron, cobalt, platinum and palladium; andconcentration of said material does not exceed 1×10¹⁹ atoms/cm³.
 8. Thetransistor of claim 7 wherein said concentration of said material is1×10¹⁵ atoms/cm³ or more.
 9. The transistor of claim 7 wherein saidsilicon film contains 1×10¹⁵ atoms/cm³ to 5 atomic % of hydrogen. 10.The semiconductor of claim 7 wherein the semiconductor contains 1×10¹⁹atoms/cm³ or less each of carbon, oxygen, and nitrogen.
 11. Thesemiconductor of claim 7 wherein the crystallization of the silicon filmis confirmed by Raman scattering spectroscopy.
 12. The semiconductor ofclaim 7 wherein the semiconductor is formed on an insulating surface.13. A thin-film transistor comprising:a silicon film containing at leastone material of nickel, iron, cobalt, platinum and palladium; aninsulating film provided on said silicon film; a gate electrode providedon said insulating film; wherein said material is introduced into saidsilicon film for crystallization thereof and removed such thatconcentration of said material do not exceed 1×10¹⁹ atoms/cm³.
 14. Thetransistor of the claim 13 wherein said removal is performed byannealing said silicon film in an atmosphere containing chlorine atoms.15. The transistor of the claim 13 wherein said removal is performed bydissolving a metal silicide in hydrofluoric acid or hydrochloric acid,said metal silicide being formed by said introducing.
 16. A thin-filmtransistor comprising:a source and a drain; wherein at least one of saidsource and said drain consists of a semiconductor film containing atleast one material of nickel, iron, cobalt, platinum and palladium; andsaid material is introduced into said silicon film for crystallizationthereof and removed such that concentration of said material do notexceed 1×10¹⁹ atoms/cm³.
 17. The transistor of the claim 16 wherein saidremoval is performed by annealing said silicon film in an atmospherecontaining chlorine atoms.
 18. The transistor of the claim 16 whereinsaid removal is performed by dissolving a metal silicide in hydrofluoricacid or hydrochloric acid, said metal silicide being formed by saidintroducing.
 19. A semiconductor comprising:a crystalline silicon film;wherein said silicon film contains at least one material of nickel,iron, cobalt, platinum; and said material is introduced into saidsilicon film for crystallization thereof and removed such thatconcentration of said material do not exceed 1×10¹⁹ atoms/cm³.
 20. Thetransistor of the claim 19 wherein said removal is performed byannealing said silicon film in an atmosphere containing chlorine atoms.21. The transistor of the claim 19 wherein said removal is performed bydissolving a metal silicide in hydrofluoric acid or hydrochloric acid,said metal silicide being formed by said introducing.